This invention relates to a method of removing silicon from waste hydrochloric acid pickling solutions which are formed during the pickling of steel stock such as steel plate, steel shapes, and steel rods.
As shown in Table 1, waste hydrochloric acid pickling solutions for steel stock (hereinunder referred to simply as "waste pickling solutions") generally contain a large quantity (100-200 g/l) of iron. It is possible to recover this iron and use it for the manufacture of ferric oxide for ferrite.
TABLE 1 ______________________________________ Composition of waste pickling solution Fe Si Al Mn ______________________________________ 100-200 g/l 30-120 mg/l 30-60 mg/l 400-700 mg/l ______________________________________
Methods for manufacturing ferric oxide from such a waste pickling solution include the roasting method, the neutralizing method, and other methods, but industrially, the roasting method is generally employed because it can recover hydrochloric acid and can also produce high-purity ferric oxide.
Table 2 shows the composition of ferric oxide which is obtained by roasting of waste pickling solution without refining of the solution. It can be seen that in addition to Fe, this ferric oxide contains metallic impurities such as Si, Al, and Mn.
Table 3 shows JIS K1462 quality specifications for "Iron oxide (III) for Ferrite". Class 1 ferric oxide in this table is the highest quality. It can be seen from this table that the quality of ferric oxide for ferrite is primarily determined by the content of SiO.sub.2. It can also be seen that except for SiO.sub.2, the ranges given in Table 2 for impurities satisfy the JIS requirements for Class 1 ferric oxide given in Table 3. The SiO.sub.2 in ferric oxide which is manufactured from a waste pickling solution comes from the waste pickling solution when the roasting method is used, and it comes from the waste pickling solution and from a neutralizing agent when the neutralizing method is used. Accordingly, in order to obtain high-quality ferric oxide which satisfies the requirements for JIS Class 1 ferric oxide, it is necessary to reduce the content of silicon in the waste pickling solution as much as possible.
TABLE 2 ______________________________________ Composition of ferric oxide made by roasting of raw waste pickling solution (wt. %) Fe.sub.2 O.sub.3 SiO.sub.2 Al Mn ______________________________________ .gtoreq.98.5 0.03-0.07 0.01-0.02 0.2-0.3 ______________________________________
TABLE 3 ______________________________________ Composition of ferric oxide according to JIS K1462 (wt. %) Class Fe.sub.2 O.sub.3 SiO.sub.2 Al Mn Ca ______________________________________ 1 .gtoreq.99.0 .ltoreq.0.01 .ltoreq.0.02 .ltoreq.0.30 .ltoreq.0.01 2 .gtoreq.98.8 .ltoreq.0.06 .ltoreq.0.02 .ltoreq.0.30 .ltoreq.0.02 3 .gtoreq.98.5 .ltoreq.0.30 .ltoreq.0.05 .ltoreq.0.30 .ltoreq.0.04 ______________________________________
Silicon exists in a waste pickling solution in the form of silicic acid (SiO.sub.2.nH.sub.2 O). Various methods have been proposed for removing silicon from waste pickling solutions, and some of these methods have been put into practice.
For example, Japanese Published Examined Patent Application No. 61-289 (1986) discloses a method of reducing the silicon content of a waste pickling solution by ultrafiltration. However, as most of the silicic acid in a waste pickling solution immediately after pickling has a small particle diameter on the order of 20.ANG., only a small amount of silicic acid can be removed by ultrafiltration, and the efficiency of removal is poor.
Japanese Published Unexamined Patent Application No. 59-111930 (1984) discloses a method for reducing the amount of silicic acid in a waste pickling solution by contacting the waste pickling solution with silica gel and adsorbing the silicic acid on the silica gel. However, only a small amount of silicic acid is actually adsorbed by the silica gel, so the efficiency of removal is poor. Furthermore, it is difficult to reuse the silica gel, so processing costs are high.
Japanese Published Unexamined Patent Application No. 59-169902 (1984) discloses a method for reducing the silica content of a waste pickling solution in which the waste pickling solution is concentrated by being contacted with a high-temperature oxygen-containing gas, soluble silicic acid in the waste pickling solution in molecular or ionic form is made insoluble, and then filtration is performed. However, this method requires a concentrating apparatus and other large equipment, so equipment costs are high and operation is complicated.
Japanese Published Unexamined Patent Application No. 58-151335 (1983) discloses a method for removing silicon by adding a high polymer coagulant to a waste pickling solution. The silicic acid in the solution is made to coagulate and is then removed by filtration. However, the silicic acid in a waste pickling solution is extremely fine at first, so little coagulation can be expected. Furthermore, the polymer coagulant can produce an adverse effect because it covers the surface of active silicic acid particles, so the spontaneous coagulation of the silicic acid which will be described below in detail may be impeded.
Japanese Published Unexamined Patent Application No. 60-122087 (1985) discloses a method in which a surfactant is added to a waste pickling solution, and silicic acid is then removed by filtration. Although the surface of silicic acid in a waste pickling solution is initially hydrophilic, when it is covered with a surfactant, it becomes hydrophobic and easier to coagulate. On the other hand, if too much surfactant is present, the surface again becomes hydrophilic and defloculation occurs, so it is necessary to add the appropriate amount of a surfactant in order to maintain a state of coagulation. However, the waste pickling solution already contains a surfactant which was added at the time of pickling of steel stock in order to prevent excessive pickling, so it is difficult to control the amount of surfactant which is added to the solution. Furthermore, just as in the case in which a polymer coagulant is added, the addition of a surfactant reduces the activity of the surface of the silicic acid in the waste pickling solution which would normally make the particles of silicic acid join to one another. Therefore, spontaneous coagulation is impeded, and the silicic acid remains in the form of minute particles which are nearly impossible to separate by filtration.